ABSTRACT Pregnancy is a condition characterized by progressive insulin resistance and impaired glucose metabolism. Gestational diabetes mellitus (GDM) is a common pregnancy complication that is defined as glucose intolerance with onset or first recognition during pregnancy. GDM affects 9% of pregnant women in the United States and its prevalence is increasing. Understanding the etiology of GDM and early identification of women at risk of GDM are imperative, because GDM is associated with risk of adverse health outcomes in both mothers and their children. As a pregnancy complication, GDM may involve not only pathways as shared with type 2 diabetes, but also pregnancy-related unique factors. The human placenta harbors a rich and diverse transcriptome, including numerous miRNA species. Recently, the findings that miRNAs of placental origin are released in the maternal circulation throughout pregnancy has raised the exciting prospect of using miRNA expression profiles as noninvasive markers of placental function during pregnancy. Pregnancy-associated miRNAs in maternal circulation may represent a channel for fetal-maternal communication. However, the role of pregnancy-associated miRNAs in the development and prediction of GDM remains unknown. In this proposed study, we aim to investigate the association between pregnancy-associated miRNAs in maternal plasma during early pregnancy and subsequent risk of GDM (Aim 1); the potential utility of pregnancy-associated miRNAs in maternal plasma samples during early pregnancy in predicting the risk of GDM (Aim 2); and the associations of pregnancy-associated miRNAs with placental hormones and GDM-related metabolic markers in maternal plasma samples (Aim 3). We will use plasma samples along with data that are already collected in the Maternal Fetal Tissue Bank at the University of Iowa. We will measure previously-identified and pregnancy-associated miRNAs in first-trimester plasma samples of 360 pregnant women (180 GDM cases and 180 matched controls). Individual miRNAs will be reverse transcribed and quantified by real-time qPCR using the TaqMan MiRNA assays. We will also measure placental hormones (human placental lactogen and human placental growth hormone) and GDM-related metabolic markers (adiponectin, C-reactive protein, ferritin, and sex hormone binding globulin) using commercially available ELISA kits. Information on maternal demographic characteristics, anthropometric measures, and clinical diagnosis of GDM will be obtained from the University of Iowa Hospitals and Clinics electronic health records. The proposed study will for the first time investigate the role of pregnancy-associated miRNAs as novel predictors of GDM. It may provide evidence to change the clinical paradigm in the screening and prediction of GDM and lead to improved understanding in the causes of GDM.